There’s a saying that when you go on traveling tours, you get to know whom the designated jerk is going to be within three days, and if you don’t know it by then, you’re the jerk.

In this novel, the person held universally in contempt is an unattractive spinster named Harriet Baxter, but she is blithely clueless in this regard. If asked, she’d probably describe herself as a model of self-knowledge, humbly aware of her shortcomings. She knows that she’s not comely (she says so several times). She knows that as a single woman, she’s not particularly welcome in any social situation. And she knows that her considerable wealth can get on people’s nerves. She counters all this with a welter of good deeds; she gives the reader to understand that she’s an indispensable friend.

In fact, she’s a monster. “Gillespie and I”is a deliciously morbid, almost smutty story, a compendium of inappropriate wants and smarmy desires. Harriet is not above smearing feces on the wallpaper just because she feels like it or — more likely — to get someone into trouble.

Harriet lives in Glasgow in the year 1888. She is a paragon of virtue. She begins her story with an account of how she recently saved the life of a respectable matron on the street, a woman who fainted and almost choked to death on her dentures. The woman she saves is Elspeth Gillespie , mother of the promising young Scottish artist Ned Gillespie . Harriet describes him as her dear friend and soul mate.

“We were connected through the most intimate of friendships,” she says. “So profound was our rapport that I was, on occasion, the first to behold his completed paintings, sometimes before his wife Annie had cast her gaze upon them.”

“Gillespie and I: A Novel” by Jane Harris (HarperPerennial)

Well, we know something is up from that statement. Harriet aspires to be a home wrecker. The Gillespie family is first and foremost just that — a family. There’s Ned; his wife, Annie; his mother, Elspeth; and two kids, Sybil, who’s about 7, and her baby sister, Rose. Little by little, and in the most casual manner, Harriet becomes what she refers to as a family friend. She’s always out walking in the most innocent way when she finds herself in the Gillespies’ neighborhood. She tags along when the family goes to the park; she befriends their other friends; she showers the children — who are a bit of a handful — with gifts.

We know all is not well with Harriet. She has a stepfather whose attention she craves but who treats her with no more than amused contempt. She ensures her welcome at the Gillespies’ house by making herself useful — or so she tells us. She helps out with their taxes (which gives her a good idea of the family’s finances). And she drops by before the Gillespies’ New Year’s party, where, strangely enough, several people get violently ill. The Gillespies begin to accept her with that controlled exasperation you feel about any domestic pest — a female version of the guy who comes over and hangs out on your couch and never, ever, goes home.

Perhaps Harriet is the reason that the kids, particularly Sybil, begin to get out of hand. Doctors are called in; her parents consider sending the girl to some kind of corrective institution. Harriet is an attentive friend through all this, finding ways to advance Ned’s career as well, and goes so far as to commission her portrait to be painted by Annie. Who among the Gillespies would venture to get rid of Harriet under these circumstances?

To say anything more would be to give away the plot, which is too delectable to spoil. I can say that there are what amounts to three different novels in these 500 pages, each one creepier than the last. If you are in any way squeamish or genteel, skip “Gillespie and I.” If you’d like to know a little more about the seamy side of the human condition, by all means, pick this one up.

See reviews books regularly for Book World.

Gillespie and I

By Jane Harris.

Harper Perennial.
528 pp. Paperback, $14.99.